1. Field of the Invention
The present invention relates to a drive method of a light-emitting display panel using, for example, organic electroluminescence (EL) elements acting as light-emitting elements and to a display device using the light-emitting display panel, and more particularly, to a light emission luminance control technology for causing, when the light-emitting display panel starts to be driven for light emission or when it is intended to increase light emission luminance while the light-emitting display panel is being driven for light emission, light emission to rise up instantly or light emission luminance to increase instantly following the above operation or intention.
2. Description of the Related Art
Attention is given to an organic EL display as a successor of a liquid crystal display because the organic EL display can reduce power consumption, can display an image of high quality and further can be reduced in thickness. This is because the efficiency and life of the organic EL display have been improved to a practically usable level by using an organic compound promising good light emitting characteristics for the light-emitting layers of EL elements used in the EL display.
There have been proposed a passive matrix drive system and an active matrix drive system as a drive method of a display panel in which the EL elements are disposed. FIG. 5 shows the passive matrix drive system and an example of the display panel whose light emission is controlled by the passive matrix drive system. Two drive methods, that is, a cathode line scan/anode line drive method and an anode line scan/cathode line drive method are available as a drive method of the EL elements in the passive matrix drive system. FIG. 5 shows the arrangement of the former cathode line scan/anode line drive method.
That is, the display panel is arranged such that anode lines A1 to An are disposed in a vertical direction as n-pieces of drive lines, whereas cathode lines B1 to Bm are disposed in a horizontal direction as m-pieces of scan lines, and organic EL elements OEL shown by the symbol of diode are disposed at the intersections (n×m positions in total) of the respective lines. Then, the respective EL elements as light-emitting elements constituting pixels are disposed in a lattice shape, and one ends thereof (anode terminals of the EL elements) are connected to the anode lines and the other ends thereof (cathode terminals of the EL elements) are connected to the cathode lines in correspondence to the intersections between the anode lines A1 to An along the vertical direction and the cathode lines B1 to Bm along the horizontal direction. Further, the anode lines are connected to an anode line drive circuit 2, and the cathode lines are connected a cathode line scan circuit 3 so as to be driven respectively.
The scan circuit 3 has scan switches Sy1 to Sym in correspondence to the respective cathode scan lines B1 to Bm. The scan switches Sy1 to Sym act to supply any one of a reverse bias voltage VM supplied from a reverse bias voltage creation circuit 5 to prevent the crosstalk light emission of the elements and a ground potential acting as a reference potential to corresponding cathode can lines. Further, the anode line drive circuit 2 has constant current circuits I1 to In for supplying drive currents to the respective EL elements through the respective anode lines and drive switches SX1 to SXn.
The drive switches SX1 to SXn act to supply any one of the currents from the constant current circuits I1 to In and the ground potential to corresponding anode lines. Accordingly, when the drive switches SX1 to SXn are connected to the constant current circuits, they supply the currents from the constant current circuits I1 to In to the respective EL elements disposed in correspondence to the cathode scan lines.
Note that it is possible to use a voltage source such as constant voltage circuits, and the like in place of the constant current circuits. However, the constant current circuits are ordinarily used as shown in FIG. 5 because of the reasons that the voltage/luminance characteristics of the EL elements are unstable to a temperature change while the current/luminance characteristics thereof are stable to the temperature change, that there is a possibility that the EL elements are deteriorated by an excessive current, and the like.
A light emission control circuit 4 including a CPU is connected to the anode line drive circuit 2 and to the cathode line scan circuit 3 through control buses, and the scan switches SY1 to SYm and the drive switches SX1 to SXn are manipulated based on the signals of an image to be displayed. With this arrangement, the constant current circuits I1 to In are appropriately connected to desired anode lines while setting the cathode scan lines to the ground potential at a predetermined cycle based on the image signals. Accordingly, the respective EL light-emitting elements selectively emit light, thereby the image is reproduced on the display panel 1 based on the image signals.
A DC output (output voltage=VH) from a drive voltage source 6 composed of, for example, a voltage increasing type DC-DC converter is supplied to the respective constant current circuits I1 to In of the anode line drive circuit 2. With this arrangement, the constant currents created by the constant current circuits I1 to In, to which the output voltage VH has supplied from the drive voltage source 6, are supplied to the respective EL elements disposed in correspondence to the anode scan lines.
In contrast, the value of the reverse bias voltage VM used to prevent the crosstalk light emission of the EL elements is ordinarily generated by series regulating the output voltage VH because the voltage VM is relatively near to the value of the output voltage VH and the current consumed by the reverse bias voltage VM is smaller than that of the output voltage VH. Thus, it is considered that the employment of the above arrangement is advantageous from the view point of the number of parts and power consumption.
A reverse bias voltage creation circuit 5 having a simple arrangement shown in FIG. 5 can be preferably employed as the series regulating circuit. The reverse voltage creation circuit 5 is composed of a voltage division circuit for dividing the output voltage VH from the drive voltage source 6 and a transistor Q1 for outputting the divided voltage created by the voltage division circuit as a reverse bias voltage by subjecting it to impedance transformation. That is, the voltage division circuit is composed of resistors R1 and R2 connected in series between the drive voltage source 6 and the reference potential (ground), and the collector terminal of the npn transistor Q1 that achieves the impedance transformation function is connected to the drive voltage source 6, and the base terminal thereof is connected to the node between the resistors R1 and R2. With this arrangement, the transistor Q1 is in an emitter follower connection, and the reverse bias voltage VM is output from the emitter terminal of the transistor Q1.
Incidentally, according to the drive unit arranged as described above, the constant current circuits are provided in correspondence to the respective anode lines to drive the respective EL elements by the constant currents. In the constant current circuits, a certain amount of voltage drop must be taken into consideration in the circuits to drive the respective EL elements by the constant voltage at all times. Accordingly, the output voltage VH from the drive voltage source 6, which is supplied to the constant current circuits, must have a voltage value equal to or larger than the voltage value obtained by adding the amount of voltage drop arisen in the constant current circuits to the forward direction voltages VF of the respective EL elements driven by the constant currents.
Moreover, when the electric dispersion and deterioration with age of the respective EL elements and further the dispersion of the respective elements in the constant current circuits are taken into consideration, it is necessary to set the output voltage VH by further adding a predetermined margin to the amount of voltage drop in the constant current circuits. When this margin is added, the amount of voltage drop is made excessive in almost all the constant current circuits, thereby a problem is arisen in that a power loss is increased in the constant current circuits.
Thus, it is contemplated to detect the forward direction voltage VF of each EL element driven by the constant current by, for example, a sampling and holding means and to control the value of the output voltage VH supplied from the drive voltage source 6 based on the thus detected forward direction voltage VF. When the control means described above is employed, it is possible to create the output voltage VH by adding a given voltage value, which can guarantee that each EL element is driven by the constant voltage in the constant current circuits, to the forward direction voltage VF. Accordingly, it is possible to set the margin to a very small amount so as to reduce the power loss in the constant current circuits. With this arrangement, when this drive method is used in, for example, mobile appliances, and the like, the power consumption of batteries can be reduced.
In contrast, it is known that the organic EL elements described above have diode characteristics including a predetermined electric capacitance (parasitic capacitance) from the laminated structure thereof. Then, when the organic EL elements are driven by constant currents as described above, the anode voltage waveform of the elements has such a characteristic that it slowly rises up as shown in FIG. 6 because the constant current circuits are high impedance output circuits in the operation principle thereof. That is, in FIG. 6, a vertical axis shows the anode voltage V of an element, and a lateral axis shows an elapsed time t.
The rising-up curve of the anode voltage V is changed by various conditions such as the lighting/non-lighting condition of the element when it was scanned last time, the lighting/non-lighting condition of an adjacent element, and the like. Then, the luminance of the organic EL element is also changed by the change of the rising-up curve. However, the substantial luminance of the display panel cannot help being dropped because the rising-up of the light emission of the element is delayed.
To cope with this problem, there is proposed a drive method of connecting a constant voltage source to an element when the element is driven for light emission and providing a precharge period during which the parasitic capacitance of the element is instantly charged. There is available a cathode reset method as a typical drive method of executing the precharge and is disclosed in, for example, JP 09-232074 A. According to this cathode reset method, it is possible to instantly rise the anode voltage of an EL element to be lit to a voltage near to the reverse bias voltage VM for preventing the crosstalk light emission by making use of the parasitic capacitance of the EL elements and the reverse bias voltage VM.
FIG. 7 shows an anode voltage waveform when a precharge voltage (VM) is set equal to the forward direction voltage (VF) of an element. A vertical axis shows the anode voltage V of an element, and a lateral axis shows an elapsed time t also in FIG. 7. In FIG. 7, a period a shows a precharge period to the element and a period b shows a constant current drive period of the element.
Then, the following problem is arisen when the precharge drive as described above is executed as well as when the control means described above is employed to obtain the forward direction voltage VF of each EL element by making use of, for example, the sampling and holding means and to control the value of the output voltage VH supplied from the drive voltage source 6 using the forward direction voltage VF. That is, when the light emission luminance of a light-emitting element, which is being lit for light emission, is increased, the forward direction voltage VF of the element increases as shown in FIG. 8. At this time, a final forward direction voltage VF cannot be sampled and held depending on timing of a sampling operation and a voltage denoted by VF′ is held based on the timing of the sampling operation, and the output voltage VH of the drive voltage source 6 is controlled based on the thus held voltage VF′.
Since the voltage VM used for the precharge is created based on the output voltage VH from the drive voltage source 6, a higher precharge voltage VM shown in FIG. 9 is created next based on the held voltage VF′ shown in FIG. 8. Accordingly, the luminance of the EL element does not increase instantly but increase stepwise as shown in FIG. 10. Thus, a problem is arisen in that the slow change of luminance as described above is felt unnatural by a user. Note that t1, t2, and t3 in FIG. 10 show timing at which a sampling operation is executed, and c shows sampling intervals.
Further, even if the precharge as described above is not executed and the light-emitting elements are driven by the constant current, luminance changes slowly likewise. That is, when light emission luminance is increased and the forward direction voltages VF are increased, the voltage VH output from the drive voltage source 6 is a previous voltage until sample hold timing for detecting the forward direction voltages VF arrives. Thus, the difference of potential between the voltage VH and the voltage VF is reduced and the constant current circuits for driving the respective light-emitting elements by the constant current cannot execute a constant current supply operation. As a result, while the luminance of the light-emitting elements increase, the predetermined luminance thereof cannot be reached.
When the sample hold timing for detecting the voltage VF arrives, the voltage VH is controlled to a higher voltage and the constant current circuits also can execute the constant current supply operation up to a higher voltage VF, thereby the luminance is increased. The repetition of the above operation causes the luminance to reach a predetermined value stepwise. With the above operation, the luminance changes slowly likewise with a result that the user has unnatural feeling. Further, this defect is arisen in the same way also when, for example, the display panel starts to be driven for light emission.
The phenomenon described above is mainly caused by the sampling intervals of the sampling and holding operation (which ordinarily operates at the intervals of several hundreds of milliseconds). Accordingly, it is conceived to execute the sampling and hold operation at timing of short intervals (for example, at the intervals of several tens of milliseconds). However, when the sampling and holding operation is executed at the timing of the short intervals at all times, a drive power necessary to the sampling and holding operation and the voltage held by the operation are discharged each time the operation is executed, thereby a power is wasted. Therefore, when this drive method is used in, for example, mobile terminals, and the like, the power of the batteries thereof is wasted, and thus this drive method is not preferable.